How to Reopen the 3D Viewport along side the UV Edit Window
I have created a hotkey for performing this most frequently needed operation with areas.


RMB click on the edge between two areas
Choose Split Area
Right Click on it again.
Choose Add Shortcut
Press the hotkey you would want to use for splitting areas (I used CTRL+ALT+SHIFT+X)
Then press the hotkey to initiate the split.
Pressing MMB button can change the direction of split.
Click the mouse when satisfied with the proportions of split.




References

Arranging Areas in Blender